haneybird

Allegiance spells restrict what units can cast them. They do not restrict what units they can be cast on. The keywords in the spell text will determine what units are valid targets.

You are correct in your reading of the rule in the book. The Azyr app should not be used as a rules reference.

To add to what RoloMcFury said, this is why some abilities say you can add models to units and some say you can return models to units. Adding models adds new models to the unit, increasing the size of the unit if you are at the original size (or larger). Returning models restores models that were originally in your unit but have been slain or ran away due to battleshock but can not increase the size of your units.

It is not a regular Keeper of Secrets and uses its own summoning value. However, since it is a Forge World unit you are technically supposed to ask your opponent's or TO's permission first before you use it so if you play fair you will warn the opponent about the summoning cost change and probably have to agree to summon it at the higher value.

The Command abilities from the Chaos Lord and the Keeper of Secrets both specify that they work on units that have attacked once and that they allow you to attack for a second time which means no triple attack for Archaon.

Destiny Dice are regular rolls that happened before the game started. A Destiny Dice that has a value of 1 is an unmodified roll of 1 for whatever you use it for. This also means that Destiny Dice are vulnerable to abilities or effects that cause a reroll.

Overall the Chariot is probably going to do more for you as it has much more utility. The biggest things the Daemon Prince has going for it is the ability to fly without costing an artifact and the "first strike" ability it has. A winged Daemon Prince is going to be best used as an assassin that you get past the other player's screening units to strike at heroes or ranged units in their backline. Ranged units are going to be our biggest threat so having something that can go after them will be useful but you probably will not want more than one and will need it to have wings.

Also, Warscroll Builder lists Castellens of the Crimson Keep and Court of Nulamia in both the Legions of Nagash and the Soulblight headings. Battalions that have been stated by GW to not be cross faction legal such as the Everchosen Fatesworn Warband are only listed in a single location, in this case Everchosen. This strongly suggests that Soulblight was meant to be able to use these battalions as the same as the Legions as they are the only ones in the LoN book that contain all Soulblight units.

Note that this does not say that the battalion can be part of your army. It is part of your Allegiance. Fatesworn Warband inherits the TZEENTCH keyword because all members of the battalion have the TZEENTCH keyword.